1. Field of the Invention
The invention relates generally to a primary coolant system of a nuclear power plant which provides coolant to a primary loop via an accumulator tank and, more particularly, to such a primary coolant system providing an initial high rate of flow to the primary loop and then providing a lower rate of flow for long term cooling of the primary loop.
2. Background of the Related Art
A typical nuclear power facility includes a nuclear reactor (typically pressurized to approximately 2250 psig) wherein a controlled nuclear reaction, which generates heat, is occurring. Typically, borated water is contained in the reactor for controlling the nuclear reaction process and for passing the heat away from the reactor. A primary loop communicating with the reactor functions to pass the borated water (i.e., the heat) away from the reactor and to transfer the heat to a secondary loop. The secondary loop is isolated from the primary loop and generates steam from the heat passed from the primary loop. The steam of the secondary loop is used to produce electricity as is well known in the art. The primary loop then returns the borated water back into the reactor where the above described process is repeated.
An accumulator tank, which is typically pressurized to approximately 800 psig, contains a coolant (typically borated water), and is attached to the primary loop for injecting coolant into the primary loop in the unlikely event of a break therein. A gas, usually nitrogen, is placed in the upper portion of the accumulator tank for maintaining a pressure in the tank. As is obvious to those skilled in the art, a break in the primary loop would cause the reactor vessel to overheat if the borated water, which transfers the heat away from the reactor vessel, is emptied from the primary loop and, in turn, the reactor vessel. The accumulator tank replenishes the coolant to the primary loop for mitigating, and possibly eliminating, this overheating.
One mechanism for injecting the coolant from the accumulator tank into the primary loop includes outlet piping attached between the accumulator tank and the reactor vessel. A valve, which is open during normal operation, and an orifice are both disposed in the outlet piping for respectively closing the outlet piping during planned maintenance of the reactor vessel and for controlling the flow of the coolant into the primary loop. A check valve, which allows flow in only one direction, is also disposed in the outlet piping and allows only the flow of the coolant into the primary loop, effectively eliminating any possibility of backflow of coolant from the primary loop into the accumulator tank during normal operation.
As will be obvious to those skilled in the art, since the reactor is at a higher pressure than the accumulator tank during normal operation, coolant does not flow from the accumulator tank into the primary loop. However if a break occurs in the primary loop, the primary loop will eventually be at a lower pressure than the accumulator tank. This allows the coolant in the accumulator tank to flow into the primary loop for replenishing the lost coolant.
Although the presently known and utilized system for injecting coolant into the primary loop if a break occurs therein is satisfactory, it is not without drawbacks. The coolant flows into the primary loop at a substantially high rate, and this causes the accumulator tank to empty at a fast rate. Therefore, long term cooling of the reactor vessel is minimal.
Consequently, a need exists for an improved primary cooling system of a nuclear power plant for providing coolant to the reactor vessel.